Paving breakers have been used in the construction industry for over a hundred years to repair and remove rigid aggregate materials such as rock, concrete, asphalt, etc. These tools employ a rapidly reciprocating tool, sometimes referred to in the industry as a moil point or steel having a chisel like end that impacts rapidly against and into the aggregate material in vibratory fashion. The moil point is usually reciprocated by a free piston slidable within a cylinder, driven by compressed air from a reversing valve, having a forward power stroke in which it impacts directly against the inner end of the moil point that projects within the cylinder. The steel is power driven only outwardly from the tool housing and either the reaction force from the work, or a retainer that engages a stop flange on the steel, returns the steel to its inner position projecting within the cylinder so that it may again be impacted by the piston during its next power stroke.
The reversing valve itself has taken several forms in the past such as an annular plate valve or a sleeve valve slidable on a valve guide stem, and these valves serve to direct compressed air to the opposite sides of the piston to cause piston reciprocation in the cylinder. These valves are shifted by differential pressure in the forward and rear cylinder chambers. The reversing valve does not usually control exhaust flow from the cylinder and this is frequently effected by an exhaust passage in the cylinder opened and closed by movement of the piston itself. When the reversing valve ports air to the backside of the cylinder during the power stroke, the piston travels forwardly (usually downwardly) in the cylinder driving air outwardly from the exhaust port in the cylinder until the piston covers the exhaust port and forward travel thereafter compresses air in the forward cylinder chamber as the steel end is approached. To prevent the compression of air in the forward cylinder chamber from unnecessarily decelerating the piston, return air passages communicating with this forward chamber frequently are formed by a plurality of large long bores in the tool housing that act as an accumulator for air being compressed in the forward chamber by the piston.
As the piston continues its forward stroke toward the steel end the rear end of the piston uncovers the exhaust port in the cylinder thereby exhausting the rear cylinder chamber reversing the differential pressure acting on the reversing valve and shifting the valve, terminating air flow to the rear cylinder chamber and porting air to the forward cylinder chamber through the return air passages. After the piston impacts against the steel end the increased pressure in forward chamber causes piston reversal into its return stroke. The piston exhaust port valving is the same in its return stroke causing differential pressure shifting of the reversing valve as the rear end of the piston uncovers the exhaust port.
Air flow to the reversing valve is conventionally controlled by a throttle lever controlled on/off valve in the tool housing adjacent tool handles. This rear housing area is conventionally referred to as a "backhead" and it usually includes two aligned outwardly extending handles--one of the handles has a throttle lever and is referred to as a "live" handle and the other handle is referred to as a "dead" handle. With this arrangement a worker can hold the tool handles with his right and left hands, operate the throttle lever and manipulate the tool without taking his hands off the handles.
Because of the high forces created in paving breaking tools of this type as a result of the impact of the piston directly against the steel end, it was heretofore believed necessary that the paving breaker housing be constructed of very heavy materials to absorb the high shock forces. Usually the housings are constructed of iron forgings and these are quite costly due to the heavy equipment required in their manufacture as well as the material cost itself. Moreover, the accumulator chambers, which form part of the return stroke air passages, must be drilled into the housing forging. Since these holes extend for a substantial distance down the length of the tool housing and since there are usually four to six such passages to provide the necessary accumulator volume, they represent a major cost factor in the manufacture of paving breaker tools.
Another problem in prior paving breakers utilizing heavy forged housing results from the thickness of the forging. Heat generation in the cylinder requires that some cooling passages be formed around the cylinder to maintain a sufficiently low operating temperature and this also adds significantly to the manufacturing cost of the tool.
The reversing valves for known paving breakers also have significant disadvantages. In the annular plate type reversing valve referred to briefly above, the plate has a relatively short axial length and is slidable in an annular chamber within mating valve block members. The outer peripheral surface of the plate controls air flow and thus requires a clearance between it and the valve block chamber for proper functional operation. However, this clearance permits the valve plate to tilt slightly as it reciprocates back and forth under differential pressure reversal imposed on it by cylinder chamber pressure. This tilting causes valve wear and contamination buildup, and eventually, if sufficient wear occurs, the valve may even lock in a canted position within the chamber rendering the tool inoperable.
Another type of known reversing valve refered to above is a sleeve type valve mounted and guided on a fixed guide post within a valve block. The valve sleeve has a long axial length to prevent valve tilting commonly associated with plate valves, and has a central outer land that meters flow with cooperating stationary annular surfaces in the associated valve block to both the forward and rear cylinder chambers as it reciprocates under differential pressure reversal. To promote the expansion of air as it passes over this metering land, two frusto-conical expansion surfaces are formed on the opposite sides thereof. Adjacent each of these metering lands are diametral, sealing surfaces that selectively engage annular sealing ribs formed within the valve block. While this prior valve has been found functionally satisfactory, it requires the accurate machining and grinding of at least nine surfaces, i.e. two annular metering surfaces and two sealing rib surfaces in the valve block, and one land, two frusto-conical expansion surfaces and two diametral sealing surfaces on the valve member. All these machining and grinding operations increase the resulting overall paving breaker cost.
A further problem in prior paving breakers results from the misalignment of the moil point with respect to the axis of the cylinder, as the steel itself is not precision mounted in the front end of the tool and in fact is rather loosely mounted so that misalignment between the tool steel and the cylinder axis frequently occurs. As the piston impacts against a misaligned tool it causes side reaction forces against the piston which tend to pivot the piston against the cylinder. Eventually this deleterious action causes unwanted cylinder wear.